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The response of wheat crops to elevated CO 2 (eCO 2) was measured and modelled with the Australian Grains Free‐Air CO 2 Enrichment experiment, located at Horsham, Australia. Treatments included CO 2 by water, N and temperature. The location represents a semi‐arid environment with a seasonal VPD of around 0.5 kPa. Over 3 years, the observed mean biomass at anthesis and grain yield ranged from 4200 to 10 200 kg ha−1 and 1600 to 3900 kg ha−1, respectively, over various sowing times and irrigation regimes. The mean observed response to daytime eCO 2 (from 365 to 550 μmol mol−1 CO 2) was relatively consistent for biomass at stem elongation and at anthesis and LAI at anthesis and grain yield with 21%, 23%, 21% and 26%, respectively. Seasonal water use was decreased from 320 to 301 mm (P = 0.10) by eCO 2, increasing water use efficiency for biomass and yield, 36% and 31%, respectively. The performance of six models (APSIM‐Wheat, APSIM‐Nwheat, CAT‐Wheat, CROPSYST, OLEARY‐CONNOR and SALUS) in simulating crop responses to eCO 2 was similar and within or close to the experimental error for accumulated biomass, yield and water use response, despite some variations in early growth and LAI. The primary mechanism of biomass accumulation via radiation use efficiency (RUE) or transpiration efficiency (TE) was not critical to define the overall response to eCO 2. However, under irrigation, the effect of late sowing on response to eCO 2 to biomass accumulation at DC65 was substantial in the observed data (~40%), but the simulated response was smaller, ranging from 17% to 28%. Simulated response from all six models under no water or nitrogen stress showed similar response to eCO 2 under irrigation, but the differences compared to the dryland treatment were small. Further experimental work on the interactive effects of eCO 2, water and temperature is required to resolve these model discrepancies.

The amounts of foliage nitrogen (N) fixed by various annual and perennial legumes growing in Australian pastures range from <10 to >250 kg N/ha.year. Differences in N2 fixation result from variations in the proportion of the legume-N derived from atmospheric N2 (%Ndfa) and/or the amount of legume-N accumulated during growth. On-farm surveys of %Ndfa achieved by legumes growing in farmers’ paddocks in Australia indicated that N2 fixation contributed >65% of the legume’s N requirements in three-quarters of the annual legumes examined, but this decreased to two-thirds of lucerne (Medicago sativa; also known as alfalfa), and half of white clover (Trifolium repens) samples. Factors such as low numbers or the poor effectiveness of rhizobial strains in the soil, water stress, high soil concentrations of N, and nutrient disorders contribute to poor nodulation and %Ndfa values <65%, but there is also evidence that the observed %Ndfa can be dependent on the legume species present, and whether the legume is grown in a pure stand or in a mixed sward. The accumulation of legume-N relates primarily to the legume content and net productivity of the pasture. For many legume species, ~20 kg of shoot-N is fixed on average for every tonne of herbage dry matter produced. Legume productivity can be influenced by (i) sowing and establishment techniques and other strategies that enhance the legume content in pasture swards; (ii) the amelioration of soil constraints; (iii) the use of new legume species (and host–rhizobial strain combinations) that are more tolerant of hostile soil environments than subterranean clover (T. subterraneum) or annual medics (Medicago spp); and (iv) the inclusion of perennials such as lucerne to offset the year-to-year variability in productivity and N2 fixation that is a common occurrence with annual legumes.

Soil salinity, sodicity, and high extractable boron (B) are thought to reduce wheat yields on alkaline soils of south-eastern Australia; however, little quantitative information on yield penalties to edaphic constraints is available. The relationships between wheat yield of a B-tolerant cultivar and soil physicochemical conditions in the Victorian Mallee were explored using ridge regression analysis, using natural variation in the field. Wheat yields in the survey ranged from 1.3 to 6.1 Mg/ha, with low yields attributed to inadequate soil water supply during pre-anthesis growth. Crop sequences, fallow–wheat, and pulse–wheat left greatest soil water prior to sowing of the wheat crop, and lucerne–wheat the least. A descriptive model explained 54% of variation in wheat yield, with rainfall around anthesis, available soil water in the 0.10–0.40 m layer, nitrate in the 0–0.10 m layer at sowing and salinity, and sodicity in the 0.60–1.00 m layers being important factors. Subsoil salinity (ECe) and sodicity (ESP) appear to be effective surrogates for estimating the likelihood of water extraction in the deep subsoil. The analyses suggest that subsoils need to have an ECe <8 dS/m and ESP < 19% for crops to make use of water deep in the profile. Although soluble B ranged from 2 to 52 mg/kg in the 0.60–1.00 m layer of the alkaline soils considered, B appeared to have little correlation with root growth, water extraction, or yield of wheat, which has been attributed to B-tolerance of the cultivar tested and/or the overbearing effect of high Na+ in these soils.